Purification of ultralow sulfur diesel fuel

ABSTRACT

The invention is a method of purifying an ultralow sulfur diesel fuel which contains polycyclic aromatic color bodies. The method comprises contacting the ULSD fuel in the liquid phase with a coal-based activated carbon adsorbent having a surface area ranging from 800 to 1500 m 2 /g and containing pores having pore size greater than 20 Å, and recovering a purified diesel product having a decreased color bodies content.

FIELD OF THE INVENTION

This invention relates to the purification of an ultralow sulfur diesel fuel.

BACKGROUND OF THE INVENTION

The presence of sulfur compounds in diesel fuel is undesirable as they result in a serious pollution problem. Combustion of diesel fuel containing these sulfur impurities results in the release of sulfur oxides which are noxious and corrosive. Federal legislation, specifically the Clean Air Act of 1964 as well as the amendments of 1990 and 1999, has imposed increasingly more stringent requirements to reduce the amount of sulfur released to the atmosphere. As a result, the United States Environmental Protection Agency lowered the sulfur standard for diesel fuel to 15 parts per million by weight (ppmw), effective in 2006.

The most common method of removing sulfur from diesel fuel is hydrodesulfurization, in which the diesel fuel is reacted with hydrogen gas at elevated temperature and high pressure in the presence of a catalyst. See, for example, U.S. Pat. No. 5,985,136. Catalysts for the production of ultralow sulfur diesel (ULSD) fuel, having less than 15 ppm sulfur and typically 4-8 ppm sulfur, include supported cobalt-molybdenum or nickel-molybdenum catalysts.

Numerous methods have been taught in the prior art for removing various impurities in hydrocarbon streams. For instance, U.S. Appl. Pub. No. 2004/0129608 discloses a process for decolorizing and removing some trace impurities such as indanes, naphthalenes, phenanthrenes, pyrene, and alkylbenzenes from fuel streams, in particular gasoline. The process comprises contacting the fuel stream with a decolorizing carbon. U.S. Pat. No. 4,977,871 discloses a system for the selective removal of polynuclear aromatic hydrocarbons containing 3 or more aromatic rings from lubricating oil. In particular, U.S. Pat. No. 4,977,871 teaches that wood and peat based carbons are significantly more effective in the removal of these polynuclear aromatic hydrocarbons. The prior art also teaches the use of promoted carbon adsorbents, which may include various metals or other promoters, for the purification and decolorization of hydrocarbon fuel. For instance, U.S. Pat. Appl. Pub. No. 2007/0184976 discloses an activated carbon useful in purification and decolorization of hydrocarbon fuel, which includes within its pore structure a polymerized phosphate. None of these prior art documents teach the purification of an ultralow sulfur diesel fuel.

In sum, new methods for purifying ultralow sulfur diesel fuel are needed. Particularly required are processes which effectively decrease the amount of polycyclic aromatic color bodies in the ULSD fuel. We have discovered an effective, convenient method to remove color bodies from ultralow sulfur diesel fuel.

SUMMARY OF THE INVENTION

The invention is a method for purifying an ultralow sulfur diesel fuel which contains polycyclic aromatic color bodies. The method comprises contacting the ULSD in the liquid phase with a coal-based activated carbon adsorbent having a surface area ranging from 800 to 1500 m²/g and pores having a pore size greater than 20 Å, and recovering a purified diesel product having a decreased color bodies content. The invention also includes a process for extending the life of a supported hydrodesulfurization catalyst used in the production of an ultralow sulfur diesel fuel. This process comprises hydrodesulfurizing a diesel fuel stream in the presence of the supported hydrodesulfurization catalyst to produce an ultralow sulfur diesel fuel having a color greater than 2.5 as measured by ASTM D 6045, and then contacting the colorized ULSD fuel with a coal-based activated carbon adsorbent having a surface area ranging from 800 to 1500 m²/g and pores having a pore size greater than 20 Å to reduce color.

DETAILED DESCRIPTION OF THE INVENTION

Ultralow sulfur diesel fuel (diesel fuel having less than 15 ppm sulfur, by weight, and preferably less than 10 ppm sulfur) is typically produced by a hydrodesulfurization method to remove sulfur from diesel fuel. The diesel fuel is reacted with hydrogen gas at elevated temperature and high pressure in the presence of a catalyst. See for example U.S. Pat. No. 5,985,136. Typical hydrodesulfurization catalysts include supported cobalt-molybdenum or nickel-molybdenum catalysts.

Ultralow sulfur diesel fuel (ULSD), as produced commercially, contains significant amounts of polycyclic aromatic hydrocarbons such as naphthalene, acenaphthalene, fluorene, anthracene, phenanthrene, pyrene, and the like (e.g., 10-30, and usually 15-20, weight percent polycyclic aromatic hydrocarbons based on the total amount of ULSD). Although the amount of polycyclic aromatic hydrocarbons in ULSD is large, only a small portion of the polycyclic aromatic hydrocarbons are polycyclic aromatic color bodies. As discussed by X. Ma et al. in Energy & Fuels, Vol. 10, 1996, p. 91-96, the polycyclic aromatic color bodies which are the major components of fluorescence color in desulfurized diesel are anthracene, fluoranthene, and their alkylated derivatives. These color bodies are first excited by UV light in the 320-400 nm range and then emit visible light in the 400-550 cm⁻¹ range.

We have found that a color problem associated with these color bodies is related to catalyst deactivation and polycyclic aromatic hydrocarbon-type color bodies in the feed to the ULSD hydrotreater. As the hydrodesulfurization catalyst deactivates with time onstream, the average bed temperature needs to be increased to maintain desulfurization activity. As the bed temperature reaches about 376° C. or above, the color of the diesel fuel increases and approaches 2.5-3 or greater (as measured by ASTM D 6045). The maximum specification for ULSD is 2.5. Ultimately, the ULSD reactor run ends when the ULSD fuel fails to meet the color specification, even though the catalyst is still performing well in terms of desulfurization. The current invention thus also includes a process to extend the life of a supported hydrodesulfurization catalyst used in the production of an ultralow sulfur diesel fuel, thus extending ULSD reactor runs.

The total amount of polycyclic aromatic color bodies contained in an ultralow sulfur diesel fuel to be treated by the process of the invention is typically greater than 30 ppm, and preferably less than 1000 ppm, giving the ultralow sulfur diesel fuel a color greater than 2.5 as measured by ASTM D 6045.

In order to reduce the level of polycyclic aromatic color bodies in the ultralow sulfur fuel, the ultralow sulfur diesel fuel is contacted in the liquid phase with a carbon adsorbent. In accordance with the present invention, the impure ultralow sulfur diesel fuel is contacted in the liquid phase with a carbon adsorbent whereby polycyclic aromatic color bodies are retained on the carbon adsorbent and a purified ultralow sulfur diesel product reduced in polycyclic aromatic color bodies content is conveniently separated.

The carbon adsorbent useful in the invention is a coal-based activated carbon adsorbent having a surface area ranging from 800 to 1500 m²/g. The coal-based activated carbon adsorbent preferably has a surface area ranging from 800 to 1200 m²/g. The coal-based activated carbon adsorbent also contains pores having a pore size greater than 20 Å (Angstrom). Preferably, at least 10 percent of the total pore volume of the coal-based activated carbon is from pores having a pore size greater than 20 Å. More preferably at least 30 percent, even more preferably at least 50 percent, and most preferably at least 90 percent, of the total pore volume is from pores having a pore size greater than 20 A. Pore volume from pores having a pore size greater than 20 Å can be measured by the BJH (Barrett, Joyner, Hallenda) Method. Molasses number is another often used method to measure the total pore volume from pores having a pore size greater than 20 Å. A high molasses number (e.g., a molasses number greater than about 200) is typically indicative of a substantial amount of pores having a pore size greater than 20 Å.

Specific commercially available coal-based activated carbons useful in the invention include Calgon Corporation's SGL® and CAL® granular carbons and NORIT® GAC 1240 and RBHG 3. We have found that coal-based activated carbon is significantly more effective than other carbons, in contrast with U.S. Pat. No. 4,977,871 which teaches that wood and peat based carbons are significantly more effective in the removal of polynuclear aromatic hydrocarbons from lubricating oil.

The coal-based carbon adsorbent may be in granular, pelleted, or powdered form. Adsorption is preferably carried out by passing the impure ultralow sulfur diesel fuel through a bed of granular carbon adsorbent or pelleted carbon adsorbent. Alternatively, powdered carbon adsorbent can be slurred in the impure ultralow sulfur diesel fuel and separated by filtration. Granular carbon adsorbent is particularly preferred.

The invention may be carried out in a continuous or batch-wise fashion in accordance with known procedures. Continuous operation is preferred, as is the use of a plurality of adsorbent contact zones. When a plurality of adsorbent contact zones is used, one zone may be in use while adsorbent in a second zone is regenerated or changed out. The use of three contact zones is particularly preferred, with two zones in use at the same time, one a lead contact zone and the second a polishing zone, while the third zone is regenerated or changed out.

The adsorptive contact is preferably carried out at moderate temperatures. Suitable temperatures are in the range of about 0° C. to 100° C., preferably 10° C. to 60° C. Flow rates of about 0.005 to 50 volumes of ultralow sulfur diesel fuel per volume of adsorbent per hour are preferred, more preferably about 0.01-0.6. In general, slower feed flow rate reduces product impurity at a given bed-volume. Therefore, flow rate may be optimized depending on the volume of adsorbent utilized in the method.

The carbon adsorbent retains the impurities adsorbed thereon and purified diesel fuel product can be separated. Initially, there can be substantially complete removal of the polycyclic aromatic color bodies and the recovered ultralow sulfur diesel fuel is of exceptional color purity. Over the course of time, the contact solids gradually become less effective for the removal of these impurities.

Thus, when the separation efficiency of the carbon adsorbent has fallen below a desired point, for instance as demonstrated by a color level greater than 2.5, the carbon adsorbent contact materials are preferably regenerated, as by contact with a heated vapor stream such as nitrogen, stream, or air at a temperature of at least 200° C. or by wash with a solvent such as mixed xylenes, methanol, acetone or water. It is advantageous to employ a plurality of parallel contact zones such that while one zone is being regenerated, the feed is passed through another zone containing fresh or regenerated adsorbent so that optimum impurities removal can be achieved.

Following the purification method, a purified diesel fuel product having a decreased polycyclic aromatic carbon bodies content is recovered. Preferably, the purified diesel fuel product has an equilibrium color in solution of less than 2.5, according to ASTM D 6045.

The invention also includes a process for extending the life of a supported hydrodesulfurization catalyst used in the production of an ultralow sulfur diesel fuel. This process comprises first hydrodesulfurizing a diesel fuel stream in the presence of the supported hydrodesulfurization catalyst until the ultralow sulfur diesel fuel produced has a color greater than 2.5 as measured by ASTM D 6045, thus above the maximum specification. The ULSD having a color greater than 2.5 is then contacted with a coal-based activated carbon adsorbent having a surface area ranging from 800 to 1500 m²/g and pores having a pore size greater than 20 Å to reduce color, and recovering an ultralow sulfur diesel product having a color less than 2.5 as measured by ASTM D 6045 in the procedure detailed above. The supported hydrodesulfurization catalyst is preferably a supported cobalt-molybdenum or nickel-molybdenum catalyst. See, for example, U.S. Pat. No. 5,985,136.

The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims.

EXAMPLE 1 Adsorption Runs with Coal-Based Activated Carbon Compared to Wood-Based Carbon

ULSD (56-58 g) having a color in solution of 2.9, as measured by ASTM D 6045, is placed in a 250-mL beaker. A coal-based carbon adsorbent (Calgon CAL® 12×40 granulated activated carbon, having surface area between 800 to 1500 m²/g, and a molasses number of 230 min.) or a wood-based carbon adsorbent (NORIT® Darco G 60/80), in varying amounts for each adsorption run, is placed in the beaker and the mixture is stirred overnight. The adsorbent is filtered (with a Buchner funnel, 3 micron filter paper) and the final equilibrium color is measured according to ASTM D 6045. The results of Examples 1A-1D using Calgon® CAL 12×40 adsorbent and Comparative Examples 1E-1H using Darco G 60/80 adsorbent are shown in Table 1.

COMPARATIVE EXAMPLE 2

ULSD (58 g) having a color in solution of 4.5-5.1, as measured by ASTM D 6045, is placed in a 250-mL beaker. Adsorbent (0.58 g), in an amount equivalent to 1 weight percent based on the amount of ULSD, is placed in the beaker and the mixture is stirred overnight. Calgon CAL® 12×40 is used for Example 2A and a carbon molecular sieve (Carbosieve® SII, 60/80 mesh, a product of Sigma-Aldrich) is used for Comparative Example 2B. The adsorbent is filtered (with a Buchner funnel, 3 micron filter paper) and the final equilibrium color is measured according to ASTM D 6045. The results are shown in Table 2. Carbosieve® SII is a molecular sieve carbon with very small pore size, the majority being less than 10 Å.

COMPARATIVE EXAMPLE 3

The procedure of Example 2 is repeated with a ULSD having a color in solution of 3.1 and with a variety of adsorbents, in amounts equivalent to 10 weight percent based on the amount of ULSD. Example 3A uses Calgon CAL® 12×40 adsorbent, Example 3B uses Calgon SGL®, Comparative Example 3C uses Amberlyst® A15, Comparative Example 3D uses Amberlyst® A35, Comparative Example 3E uses kaolin clay, Comparative Example 3F uses fuller's earth clay, and Comparative Example 3G uses silica gel. The results are shown in Table 3.

The results show that coal-based carbon adsorbents are much more effective than wood-based carbon adsorbents (and other adsorbents) at removing color bodies from ULSD. The results also show that it is important that the adsorbent contains pores having a pore size greater than 20 Å to be effective at removing color bodies. This indicates that too small a porosity is ineffective when trying to adsorb large (tri- and tetracyclic) color bodies.

TABLE 1 Adsorption Run Data for Coal vs Wood-based Carbons Amount Amount Wt. % Adsorbent ULSD Adsorbent Final Change in Total Color Run (g) (g) in ULSD Color Color¹ Change² 1A 5.8 58 10 0.4 2.5 25 1B 0.58 58 1 0.9 2 200 1C 0.0583 58 0.1 1.6 1.3 1293 1D 0.01 58 0.0172 2 0.9 5220 1E* 5.8 58 10 0.6 2.3 23 1F* 0.582 58 1 1.2 1.7 169 1G* 0.05 56.37 0.089 1.9 1 1127 1H* 0.01 56.63 0.0177 2.5 0.4 2265 ¹Change in Color = Initital ULSD Color (2.9) − Final Color (post-adsorbent). ²Total Color Change = Change in Color × Amount ULSD/Amount Adsorbent. *Comparative Example

TABLE 2 Adsorption Run Data for Coal-based Carbons vs Carbon Molecular Sieve ULSD Change in Run Feed Color Final Color Color¹ 2A 4.5 2 2.5 2B* 5.1 5.2 −0.1 ¹Change in Color = Initial ULSD Feed Color − Final Color (post-adsorbent). *Comparative Example

TABLE 3 Adsorption Run Data for Coal-based Carbons vs Carbon Molecular Sieve Change in Run Final Color Color¹ 3A 0.4 2.7 3B 0.5 2.6 3C* 2.1 1 3D* 2.3 0.8 3E* 1.6 1.5 3F* 0.9 2.2 3G* 1.8 1.3 ¹Change in Color = Initial ULSD Color (3.1) − Final Color (post-adsorbent). *Comparative Example 

1. A method of purifying an ultralow sulfur diesel fuel containing polycyclic aromatic color bodies, which comprises contacting the ultralow sulfur diesel fuel in the liquid phase with a coal-based activated carbon adsorbent having a surface area ranging from 800 to 1500 m²/g and containing pores having pore size greater than 20 Å, and recovering a purified diesel product having a decreased polycyclic aromatic color bodies content.
 2. The method of claim 1 wherein the ultralow sulfur diesel fuel has a color greater than 2.5 as measured by ASTM D
 6045. 3. The method of claim 1 wherein the purified diesel product has a color less than 2.5 as measured by ASTM D
 6045. 4. The method of claim 1 wherein the carbon adsorbent has a surface area ranging from 800 to 1200 m²/g.
 5. The method of claim 1 wherein at least 10 percent of the total pore volume of the carbon adsorbent is in pores having a pore size greater than 20 Å.
 6. The method of claim 1 wherein at least 30 percent of the total pore volume of the carbon adsorbent is in pores having a pore size greater than 20 Å.
 7. The method of claim 1 wherein the carbon adsorbent is a granulated activated carbon.
 8. The method of claim 1 wherein the contacting is performed at a temperature within the range of about 10° C. to 60° C.
 9. A process for producing an ultralow sulfur diesel fuel, comprising: (a) hydrodesulfurizing a diesel fuel stream in the presence of the supported hydrodesulfurization catalyst to produce an ultralow sulfur diesel fuel having a color greater than 2.5 as measured by ASTM D 6045; (b) contacting the ultralow sulfur diesel fuel having a color greater than 2.5 in the liquid phase with a coal-based activated carbon adsorbent having a surface area ranging from 800 to 1500 m²/g and containing pores having pore size greater than 20 Å; and (c) recovering an ultralow sulfur diesel product having a color less than 2.5 as measured by ASTM D
 6045. 10. The method of claim 9 wherein the carbon adsorbent has a surface area ranging from 800 to 1200 m²/g.
 11. The method of claim 9 wherein at least 10 percent of the total pore volume of the carbon adsorbent is in pores having a pore size greater than 20 Å.
 12. The method of claim 9 wherein at least 30 percent of the total pore volume of the carbon adsorbent is in pores having a pore size greater than 20 Å.
 13. The method of claim 9 wherein the carbon adsorbent is a granulated activated carbon.
 14. The method of claim 9 wherein the contacting is performed at a temperature within the range of about 10° C. to 60° C. 